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Dating the Tree of Life

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Some of the most basic questions about the evolution of life concern the chronology of events. When did a given taxon appear? When did it diversify? Was its diversification slow and gradual, or did it occur in bursts (evolutionary radiations), and if so, when were these bursts, and what caused them? Answering ...

Some of the most basic questions about the evolution of life concern the chronology of events. When did a given taxon appear? When did it diversify? Was its diversification slow and gradual, or did it occur in bursts (evolutionary radiations), and if so, when were these bursts, and what caused them? Answering such questions is important not only to satisfy our intellectual curiosity about the history of life, but also to allow sophisticated analyses in other fields.
Dating the TOL has become a major goal of biological research, as shown by various well-funded initiatives (at least in the USA), such as the Assembling the Tree of Life program of the National Science Foundation, which has distributed over 57 000 000 $US since 2006. Some molecular systematics laboratories now specialize in this task. This interest in dating the TOL is not surprising because beyond the intrinsic goal of reconstructing the history of taxonomic diversification, timetrees are required in many types of comparative analyses. In fact, time data are so useful that it some authors have suggested to systematically add this information to taxon names.
These advances in evolutionary biology, which require timetrees, have revolutionized modern science by allowing more rigorous analyses. For instance, conservation studies previously used species counts (at best) or higher taxon count (of genera, families...) to assess the biodiversity of various regions), but all these approaches are problematic, to various extents. There is no universally accepted species concept, despite repeated attempts to unify species concepts; as such, the entities that we call “species” and that are registered into various databanks are not ontologically comparable; some are clades, some are reproductive communities, some are evolutionary lineages, others are phenetic clusters, and yet others belong to two or more of these categories. Taxa belonging to higher categories are even more problematic in conservation biology because, like species, they share no objective properties, and they often differ rather strongly in many important aspects, such as geological age of origin, number of included species, or phenotypic variability. The most satisfactory solution that has been advocated (and widely applied) to circumvent all these problems is to use the phylogenetic diversity (the sum of branch lengths), as this captures the sum of unique history of a given clade, or of a given fauna or flora of a region.
In comparative biology, timetrees have become essential since we have realized that comparative data are not statistically independent from each other. This is because closely related taxa tend to resemble each other more closely than distantly related taxa. Thus, all modern comparative techniques for inter-specific datasets incorporate phylogenetic information, ideally in the form of timetrees.
Various evolutionary problems, such as the search of a correlation between past climatic changes or geological events (i.e. the break-up of Pangea) and taxonomic or phenotypic diversification, require timetrees to be meaningfully studied. Thus, timetrees are at the core of several biological fields, so any project that would significantly improve how they are built would benefit a large segment of the biological community.
Historically, dating the Tree of Life (TOL) has relied initially almost exclusively on the fossil record. More recently, this has been complemented increasingly by molecular timetrees, a change allowed by the tremendous growth of molecular phylogenetics in the last two decades, even though the roots of molecular dating hark back to the 1960s. This research topic will host reviews of recent methodological developments in dating the TOL or of the substantial progress that has been made recently in dating the evolutionary radiation of some taxa, as well as articles describing ne


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